Generally, in an MRI system, a substantially uniform main magnetic field, B0, is created to cover the entire region of the subject being imaged. The main magnetic field aligns magnetic dipoles of protons within the main magnetic field. Thereafter, a transient RF pulse is introduced that causes the proton dipoles to temporarily flip. Once the RF energy from the RF pulse is removed, the dipoles flip back to their relaxed state and release the energy absorbed from the RF pulse in the form of a photon having some predictable radio frequency. The photons are captured and processed to enable imaging.
Generally, the transient RF pulse is transmitted by an RF coil. One type of RF coil commonly used in MRI is known as the “birdcage coil.” For example, respective examples of birdcage coils are disclosed by U.S. Pat. No. 4,680,548 to Edelstein et al., titled “Radio Frequency Field Coil For NMR,” the entire content of which is incorporated herein by reference, and U.S. Patent Application Publication 2006/0033497 to Chmielewski et al., titled “Degenerate Birdcage Coil and Transmit/Receive Apparatus and Method For Same,” the entire content of which is incorporated herein by reference. Typically, a birdcage coil is cylindrical in shape and includes two conductive end loops or rings interconnected by an even number of rungs or axial conductors that divide the two end rings into arcs or segments defined therebetween. This construction gives this type of RF coil the appearance of a birdcage, and hence the name “birdcage coil.”
Notwithstanding the birdcage coils discussed above, there remains a desire for further improvements. In particular, when MRI technology is applied to the field of radiotherapy, traditional birdcage coils are not ideally suited for accommodating a radiotherapy system. U.S. Pat. No. 7,907,987 to Dempsey, titled “System for delivering conformal radiation therapy while simultaneously imaging soft tissue,” the entire content of which is incorporated herein by reference, discloses an example of such an MRI guided radiotherapy system.